The image below is of the page for Shumsk in the Wojewodztwo Wolynskie (Volhynia Province) Business Directory of 1923, organized by type of business. The directory, which includes both Jews and non-Jews, is in both Polish and French. Surnames are rendered in their Polish spellings.
To see additional business directories from this period under Polish rule, you can click here for the 1928 directory, here for the 1929 directory, and here for the 1930 directory (thanks to Enid Cherenson).
For those unfamiliar with Polish spellings, just follow these rules of pronunciation: Polish C is pronounced like English TZ. Polish CZ is pronounced like English CH. Polish J is pronounced like English I. Polish SZ is pronounced like English SH. Polish W is pronounced like English V. Thus, the name spelled "Kac" in Polish is equivalent to the English "Katz." Similarly, "Szmulowicz" would be written "Shmulovich" in English.